The present invention relates to a color cathode ray tube and, more particularly, to a color cathode ray tube of which an electron gun assembly is improved to obtain high resolving power on the entire surface of a phosphor screen.
In a color cathode ray tube, three electron beams emitted from an electron gun assembly are deflected in the horizontal and vertical directions to scan a phosphor screen, thereby displaying an image on the screen. In particular, in a self convergence type inline color cathode ray tube, an inline type electron gun assembly having three electron guns lined up in line on one horizontal plane is incorporated in the neck. As shown in FIG. 1A, a horizontal deflecting magnetic field is formed in a pincushion shape 1H, and as shown in FIG. 1B, a vertical deflecting magnetic field is formed in a barrel shape 1V. Non-uniform magnetic fields are formed as the deflecting magnetic fields in this manner, so that three beams self-converge toward the screen easily without requiring a special unit or the like. Currently, the color cathode ray tube of this type is the main stream.
In this cathode ray tube, since the deflecting magnetic fields described above are non-uniform, even if the beam spot at the central portion of the phosphor screen forms a true circle, the electron beam spots on the peripheral portion of the phosphor screen are under-focused in the horizontal direction as they diverge, and are over-focused in the vertical direction as they converge.
When the amount of deflection of the electron beam increases, the distance from the electron gun assembly to the phosphor screen increases. Even if the beam spot forms a small-diameter true circle at the central portion of the phosphor screen, the beam spots on the peripheral portion of the phosphor screen become over-focused.
As a result, the beam spots on the peripheral portion of the phosphor screen are greatly over-focused in the vertical direction due to the two functions described above, and are substantially focused in the horizontal direction since the two functions described above compensate for each other. More specifically, on the peripheral portion of the phosphor screen, astigmatism is generated by the difference in focused state between the horizontal and vertical directions. As shown in FIG. 2, a beam spot 2 is distorted into a noncircular shape composed of a high-luminance core 3 and a low-luminance halo 4, to considerably degrade the resolving power on the peripheral portion of the phosphor screen.
In order to improve the electron beam diameter, it is important to increase the hole diameters of the electrodes forming the main lens of the electron gun assembly, thereby decreasing spherical aberration. For this purpose, the gap among the three electron beams must be increased. When, however, the gap among the three electron beams is increased, the convergence characteristics of the three electron beams suffer. The hole diameters of the electrodes forming the main lens are limited by the inner diameter of the neck where the electron gun assembly is arranged. More specifically, as described above, to improve the resolving power of the color cathode ray tube, the main lens diameter must be increased without increasing the gap among the three electron beams, and over focus in the vertical direction on the peripheral portion of the screen must be removed.
As a method of achieving an increase in diameter of the main lens and improvement in deflection distortion, Jpn. Pat. Appln. KOKAI Publication No. 64-38947 which corresponds to U.S. Pat. No. 4,897,575 proposes an electron gun assembly having the following structure. In this electron gun assembly, as shown in FIGS. 3A and 3B, the main lens is constituted by a focusing electrode G5, two intermediate electrodes Gm1 and Gm2, and a final accelerating electrode G6. In the electron gun assembly shown in FIGS. 3A and 3B, a high voltage applied to the final accelerating electrode G6 is resistance-divided by a resistor T mounted running along the electrodes of the electron gun assembly to generate first and second predetermined voltages. The first and second predetermined voltages are applied to the intermediate electrodes Gm1 and Gm2. A voltage obtained by superposing a parabolic dynamic voltage, which changes in synchronism with the deflection of the electron beams, to a constant DC voltage is applied to the focusing electrode G5. All the electron beam holes of the focusing electrode G5, intermediate electrodes Gm1 and Gm2, and final accelerating electrode G6 which form the main lens of the electron gun assembly are true-circular holes, and the focusing electrode G5 and final accelerating electrode G6 do not have side wall portions, i.e., peripheral rims, along the surfaces of the electron beam holes. Therefore, an electric field common for the three beams is formed horizontally in the focusing electrode G5 and final accelerating electrode G6. Accordingly, a first quadrupole lens having a relatively strong focusing function in the vertical direction is formed near the focusing lens G5, and a second quadrupole lens having a relatively strong divergent function in the vertical direction is formed near the final accelerating electrode G6.
In the electron gun assembly having the above arrangement, the intermediate electrodes Gm1 and Gm2 can form an extended electric field lens, which is an extension of the main lens. Furthermore, when electron beams are deflected toward the peripheral portion of the screen, since a higher voltage (dynamic voltage) is supplied to the focusing electrode G5 to reduce the voltage difference between the focusing electrode G5 and the adjacent intermediate electrode Gm1, the function of the first quadrupole lens is weakened. The electron beams therefore diverge in the vertical direction to compensate for over-focusing in the vertical direction effected by the non-uniform magnetic fields of the deflecting yoke.
Accordingly, with the electron gun assembly having the above arrangement, the two problems, i.e., an increase in diameter and improvement in resolving power degraded by deflection distortion, can be solved.
In the electron gun assembly having the above arrangement, however, since the focusing electrode G5 and final accelerating electrode G6 of the main lens do not have side wall portions (peripheral rims) along the surfaces of the electron beam holes, the diameter in the vertical direction is decreased compared to that in the horizontal direction. Accordingly, the spherical aberration in the vertical direction becomes very large as compared to that in the horizontal direction. The electron beam spot diameters in the vertical direction increase to be larger than the electron beam spot diameters in the horizontal direction. Then, the electron beam spot becomes vertically elongated at the central portion of the screen to degrade the resolving power there.
In particular, when the size and deflecting angle of the cathode ray tube are large, the function of the first quadrupole lens described above must be reinforced. In this case, the diameter in the vertical direction must be further decreased by, e.g., changing the true-circular holes formed in the focusing electrode G5 and final accelerating electrode G6 to horizontally elongated holes. As a result, the spherical aberration in the vertical direction further increases, and the electron beam spot becomes more vertically elongated at the central portion of the screen to considerably degrade the resolving power at the central portion of the screen.
As described above, in order to improve the resolving power of the cathode ray tube, the diameter of the main lens must be increased without increasing the gap among the three electron beams, and the over focus in the vertical direction on the peripheral portion of the screen must be reduced.
As an electron gun assembly that achieves increase in diameter of the main lens and improvement of the deflecting distortion, the following one is available. In this electron gun assembly, the main lens is constituted by a focusing electrode, an intermediate electrode to which a desired voltage is applied from a resistor incorporated in a tube, and a final accelerating electrode. An asymmetric focusing electric field having a relatively strong focusing function in the vertical direction is formed near the focusing electrode. An asymmetric divergent electric field having a relatively strong divergent function in the vertical direction is formed near the final accelerating electrode. The asymmetric focusing and divergent electric fields are substantially separated from each other by the intermediate electrode. A dynamic voltage that changes in synchronism with deflection of the electron beam is supplied to the focusing electrode.
With this structure alone, the spherical aberration in the vertical direction becomes very large compared to that in the horizontal direction, and the electron beam spot diameter in the vertical direction becomes larger than that in the horizontal direction. This forms a vertically elongated electron beam spot at the central portion of the screen, and lowers the resolving power at the central portion of the screen. In particular, when the size of a cathode ray thin tube or the deflecting angle is large, the spherical aberration in the vertical direction further increases to considerably degrade the resolving power.